Production methods of GaN substrate which have been attempted so far are roughly classified to four. The first is a melt growth method, which comprises growing bulk crystal of GaN from a GaN melt under high pressure, but has not be technically established. Further, it is not known whether growth of a large crystal substrate of 1 inch or more is possible in the future or not.
The second is a solution growth method, which comprises dissolving GaN in a proper solvent (e.g., sodium) to prepare a solution, and growing the bulk crystal therefrom, but has not been technically established yet.
The third method comprises growing a GaN thick film on a sapphire substrate by CD (chemical vapor deposition) or MOCVD (metal organic chemical vapor deposition), and then detaching the GaN film by a lift-off method using laser. According to this method, formation of a large substrate is possible because the substrate size is determined depending on the sapphire substrate, but a low yield is the bottleneck.
The fourth method comprises growing a GaN thick film on a GaAs substrate by CVD or MOCVD, and thereafter melting the GaAs substrate with an acid or the like, thereby producing a GaN substrate. According to this method, also, formation of a large substrate is possible, but a high cost is the bottleneck.
Fifthly, it has been also adapted to form a ZnO layer on a sapphire substrate, generate a GaN layer on the ZnO layer, and then melt the ZnO layer to take out a GaN substrate.
In this case, it is known that use of a MgO buffer layer on sapphire is effective in improving the surface shape of the ZnO layer grown on the buffer layer (refer to Non-patent Literatures 1 and 2).
[Non-Patent Literature 1]
Chen Y, Ko H J, Hong S K, and Yao T. Layer-by-layer growth of ZnO epilayer on Al2O3 (0001) by using a MgO buffer layer, Appl. Phys. Lett., 2000; 76: 559
[Non-Patent Literature 2]
Chen Y, Hong S K, Ko H J, Kirshner V, Wenisich H, Yao T, Inaba K, and Segawa Y. Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch, Appl. Phys. Lett., 2001; 78: 3352.